Why Does a Human Baby Need a Full Year Before Starting to Walk?

Why does a human baby need a full year before it can start walking, while a newborn foal gets up on its legs almost directly after birth? Scientist have assumed that human motor development is unique because our brain is unusually complex and because it is particularly challenging to walk on two legs. But now a research group at Lund University in Sweden has shown that human babies in fact start walking at the same stage in brain development as most other walking mammals, from small rodents to elephants.

The findings are published in the journal PNAS.

The Lund group consists of neurophysiologists Martin Garwicz and Maria Christensson and developmental psychologist Elia Psouni. Contrary to convention, they used conception and not birth as the starting point of motor development in their comparison between different mammals. This revealed astonishing similarities among species that diverged in evolution as much as 100 million years ago. — Humans certainly have more brain cells and bigger brains than most other terrestrial mammalian species, but with respect to walking, brain development appears to be similar for us and other mammals. Our study demonstrates that the difference is quantitative, not qualitative, says Martin Garwicz.

Based on knowledge about development in other mammals it is therefore possible to actually predict with high precision when human babies will start to walk. This is a very unexpected and provocative finding.

The notion that humans have a unique position among mammals is not only deeply rooted among lay people, but is also reflected in fundamental assumptions in different research fields related to human development and human brain evolution.

“Our study strongly contradicts this assumption and thereby sheds new light on theories in, for instance, evolutionary and developmental biology,” says Martin Garwicz. “On the other hand, our findings fit well with the substantial similarities between the genomes of different mammals. Perhaps these similarities are after all not that surprising — although the end products ‘human' and ‘rat' may be very different, our study suggests that the building blocks and principles for how these building blocks interact with one another during development could be the same.”

The study originated in an attempt by the group to translate behavioral milestones of motor development between two distantly related species. The similarities in relative developmental time courses between the two species were so striking that the scientists started to wonder whether the regularity applied to other mammals and ultimately also to humans.

The Lund group has now compared 24 species, which together represent the majority of existing walking mammals. Some, like the great apes, are closely related to us evolutionarily while others, such as rodents, hoofed animals, and elephants, diverged from our evolutionary path about 90-100 million years ago.

Despite this, and regardless of differences in various species' brain and body size, gestation time, and brain maturity at birth, the comparison shows that the young from all species start walking at the same relative time point in brain development. Humans may be unique, but not in this particular way. When the nervous system has reached a given level of maturity, you learn to walk, whether you are a hedgehog, a foal, or a human baby…